Tandy 1000
inter
Output
Control
Signal
Definition
8255A18255A·5
Technical Reference Manual
MODE 1 (PORT A)
OBF (Output
Buffer
Full
F/F). The OBF
output
will
go
"low"
to indicate that the CPU has
written
data
out
to
the specified port. The OBF F/F
will
be
set by the rising
edge of the
WR
input and reset by ACK Input being low.
ACK
(Acknowledge Input). A
"low"
on
this
input
informs
the
8255A
that the data from port A or port B has been ac·
cepted.
In
essence, a response from the peripheral
device indicating that it has received the data
output
by
the
CPU.
INTR (Interrupt Request). A
"high"
on this
output
can be
used
to
interrupt the CPU when
an
output
device has ac-
cepted data
transmitted
by the CPU. INTR is set when
ACK is a
"one",
OBF is a
"one"
and INTE is a
"one".
It
is
reset by the falling edge
of
WR.
INTE A
Controlled
by
bit
set/reset
of
PCG.
INTE B
Controlled
by
bit
setlreset
of
PC2.
Figure
10.
MODE
1
Output
Figure
11.
Mode
1
(Strobed
Output)
6-175
AFN.Q0744C